Integrated circuit substrates have generally been formed of silicon because silicon can be grown virtually dislocation free. It is necessary for an integrated circuit to have a substrate that is virtually dislocation free because it is generally impossible to provide active semiconductor devices that operate correctly in a substrate volume having a dislocation. This is applicable to threading dislocations, i.e., dislocations which intersect a face of the substrate on which active semiconductor devices are located, as well as dislocations that extend parallel to the face containing active semiconductor devices. Threading dislocations have a tendency to collect active device materials deposited on the substrate face. The dislocations which extend parallel to the face have an adverse effect on diffused materials forming junctions semiconductor active devices.
While silicon is when properly grown, virtually dislocation free, and has been satisfactorily employed as a bulk substrate for integrated circuits, there are certain disadvantages associated with the use of silicon as a bulk substrate for an integrated circuit. The mobility of silicon is relatively low, thereby limiting the operating speed of devices fabricated on silicon substrates. In addition, it is desriable in certain types of devices to tailor a bulk substrate band gap which frequently can not be done with pure dislocation free silicon substrates.
Seiki et al, in an article entitled Impurity Effect On Grown-In Dislocation Density of InP and GaAs crystals, Journal of Applied Physics, Volume 49, No. 2, February, 1978, Pages 822-828 indicates that dislocation density in indium phosphide crystals is reduced by doping sulfur or tellurium into the indium phosphide polycrystalline starting materials. Seiki et al also states that dislocations in gallium arsenide can be reduced by doping suphur, tellurium, aluminum or nitrogen into gallium arsenide polycrystalline starting materials. Jacob further reports in the Journal of Crystal Growth, Volume 59, 1982, Pages 669-671, on doping gallium arsenide with nitrogen to reduce dislocation density. Jacob indicates that the technique employed by Seiki et al reduced dislocation density only in small diameter crystals and only in the central part of a boule. Seiki et al apparently originally believed that alloying zinc would replace dislocations but found experimentally that this is not useful for gallium arsenide crystals.
The number of dopant atoms added by Seiki et al to indium phosphide and gallium arsenide was a small fraction of one-percent of the number of atoms in the indium phosphide and gallium arsenide. Seiki et al indicates that the carrier concentration of the dopants added to the indium phosphide crystals was in the order of 10.sup.17 -10.sup.19 dopant carriers per cubic cm; the dopant concentrations added to the gallium arsenide crystals were between approximately 5.times.10.sup.17 -2.times.10.sup.19 carriers per cubic cm. The maximum molecular fractions of the alloyed dopants in these situations are x=0.001 and x=0.002, respectively. Jacob added gallium nitride to gallium arsenide crystals, such that the gallium nitride concentrations were 1.2.times.10.sup.-4 -8.times.10.sup.-3 grams per gram of gallium arsenide crystals. All of the foregoing doping effects result in molecular percentages of the dopant to the crystalline material that are a fraction of one percent.
Seiki attributed the reduced dislocations resulting from addition of dopants to the bonding strength of the dopant atoms relative to the bonding strength of the host atoms. I have found that this assumption is incorrect and accounts for the reason why Seiki et al was unable to obtain reduced dislocations by adding zinc to gallium arsenide. The bond strength between the nearest neighboring bonded atoms of a crystalline material is composed of the ionic, covalent and metallic forces which bond the atoms together. The ionic bond is not rigid but is free to turn, in contrast to the rigid covalent bond. Because the ionic bond is not rigid, it does not contribute to the shear modulus of the material which is an important factor concerning dislocation density. Thus, some crystals can have very strong ionic bonds between adjacent atoms, but have a high dislocation density because of the ability of ionic bonds to turn. To minimize dislocations, it is necessary to consider all three forces which bond adjacent atoms together, viz: the ionic bond, the covalent bond, and the metallic bond; the latter bonds are much more important than the former.
It is, accordingly, an object of the present invention to provide a new and improved semiconductor circuit structure having a substrate having virtually no dislocations over a large surface area and volume on which at least several active semiconductor devices are located.
A further object of the invention is to provide a new and improved high speed semiconductor structure.
Still another object of the invention is to provide a new and improved semiconductor structure having a virtually dislocation free, wide band gap substrate on which can be located power transistors that are likely to operate at high temperatures.
A further object of the invention is to provide a new and improved semiconductor structure having a virtually dislocation free substrate which is capable of containing very high speed electronic active elements.